Rotating devices, such as cooling fans and disk drives, in a computer system emit acoustic noise that is undesirable. Cooling fans in general generate a periodic noise known as a blade passing frequency (BPF), a noise that is generated at the tip of blades.
The acoustic noise can be disturbing and even damaging in environments such as datacenters, which contain many high performance fans. The acoustic noise has also been found to be highly distracting in quiet environments like a home theater where, for example, a media computer is deployed.
A conventional way to remove fan noise has been through the use of passive noise control mechanisms. One conventional, passive noise control mechanism contains no fans but instead uses relatively large amounts of copper, heat pipes, heat sinks, etc. to adequately cool computer system components. However, due to the amount of materials required to implement the passive noise control mechanism, such a solution has often been expensive to implement.
Another conventional, passive noise control mechanism uses specially designed large and low-speed fans to shift the BPF into lower frequency bands, where the fan noise is less disturbing to human ears. Still another conventional way to passively control noise has been through the use of suitable noise absorbing materials and mounting components with suitable fasteners to reduce vibration and noise and thus avoid the so-called “tuning fork effect.” In general, the use of noise absorbing materials tends to be more effective at reducing higher frequency noise components, but less effective at eliminating lower frequency noise.
Yet another conventional way to passively control noise has been through careful selection and placement of individual components to reduce unwanted noise. For example, the use of low-noise cooling fans with precision low noise bearings and tuned blade shaping has become popular.
While the above discussed conventional, passive noise control mechanisms are available, they are often costly and/or ineffective.
A further approach at reducing fan noise is through active noise control (ANC), which is a technique used to reduce noise and vibrations emanating from electronic devices, such as projectors and large printers, machinery, air ducts and other industrial equipment. An example of a conventional ANC system 100 is depicted in FIG. 1. As shown, the ANC system 100 includes a reference microphone 120 to detect a noise. The reference microphone 120 is connected to a control electronics 130, which is connected to a speaker 150. The speaker 150 provides anti-noise to reduce/counter the noise detected by the reference microphone 120. The ANC system 100 also includes an error microphone 140, which is used to detect the result of the noise-reduction and provides the detected result to the control electronics 130. The control electronics 130 may use the result received from the error microphone 140 to vary operation of the speaker 150.
Conventional forms of ANC have been applied to certain consumer devices, the most popular being noise canceling headphones, where the external noise is reduced within the controlled zone of each ear-cup. Other applications where ANC has been applied include air-conditioning ducts, projectors, and large printers. However, in general, implementation of ANC in such systems is difficult because of the algorithmic complexity of the ANC and additional costs incurred with increases in the size of the enclosures housing the apparatuses. The more open the solution space and thus the size of the noise field being reduced, the less effective ANC becomes and the algorithmic complexity and costs also increase.
Although there have been recent attempts to reduce noise generated in electronic devices, such attempts have proven to be less than successful because the proposed solutions are inefficient in managing power and/or have inefficient ANC algorithms.